1. Field of the Invention
The invention relates to a process for producing high-temperature resistant polymers in powder form as well as to a plant for carrying out the process.
2. Description of the Related Art
Such powders are required at the production of shaped bodies from high-temperature resistant polymers by hot pressing, which polymers,. in particular, comprise polyimides (cf., for instance, H. G. Elias and F. Vohwinkel, "Neue polymere Werkstoffe fur die industrielle Anwendung: Struktur, Synthese, Eigenschaften, Verarbeitung; 2nd Series, pp. 257 to 288, Carl Hanser Verlag Munich Vienna, 1983).
According to a process proposed in the U.S.A. and known to Applicants, the polymer solution is pressed through a spinneret and the continuous filaments formed are cut into pieces by means of a rotating scraper. Upon washing and drying, the pieces are ground to a powder. This involves the difficulty that the solvent can be removed from the cut filament pieces only by a long-lasting extraction procedure, which renders the continuous production of a powder unfeasible. In addition, long extraction and drying times impair the quality of the polymers.
In DE-C 21 43 080, mixed polyimides are described, which may be obtained in powder form by precipitation from their solutions in polar organic solvents. The products discontinuously incurring in this manner all exhibit a poor thermal conductivity and a high affinity to aprotic polar solvents as well as to water. Therefore, it is necessary to extract the precipitated polymers discontinuously over several hours and to finally dry the same under vacuum at a very high temperature of about 200.degree. C. for several hours. However, the mixed polyimides described are characterized by a sensibility to hydrolysis, in particular at elevated temperatures, for which reason defects must be put up with in view of the desired product qualities.
It is, furthermore, known to obtain fibrids, i.e., branched fibrous particles of various synthetic polymers, by applying shear forces while cooling or precipitating or by evaporating the liquid phase. Such polymer particles morphologically resemble cellulose fibers in terms of size and shape.
In EP-A - 0 131 085, for instance, the production of fibrids from thermoplastic synthetic materials by relaxation evaporization of a polymer solution under shear inducing conditions is described, with the actual or apparent solution of the polymer being relaxed in a binary spinneret by applying overheated steam.
Due to their structure, fibrids are not at all suited for the production of polymer powders with approximately spherical or cubic shapes of their individual particles. When grinding fibrids, particles again will be obtained that have fibrous structures and are not suited for pressing. Add to this that fibrids are to be comminuted with a poor grinding efficiency.
Besides, the production of fibrids from high-temperature resistant polymers has not become known yet.